Hybrid vehicles come in different forms, may use different energy storage devices, and serve different customer needs. Existing hybrid vehicles include hybrid electric vehicles (HEV), which use batteries as the energy storage system. The plug-in hybrid electric vehicle (PHEV) is an extension of existing hybrid electric vehicle (HEV) technology. A PHEV utilizes a larger capacity battery pack than a standard hybrid vehicle, and adds the capability to recharge the battery from a standard electrical outlet to decrease fuel consumption and to further improve the fuel economy in an electric driving mode or in a blended driving mode. There are also battery electric vehicle (BEV) applications where an electric machine completely replaces the internal combustion engine.
Battery state of charge (SOC) is defined as percentage of available charge as compared with the maximum charge capacity. For a battery with capacity Q, charge/discharge efficiency η, and current I:
                                                        ⅆ              S                        ⁢                                                  ⁢            O            ⁢                                                  ⁢            C                                ⅆ            t                          =                  -                                    η              ⁢                                                          ⁢              I                        Q                                              (        1        )            
By convention, current is positive when flowing out (discharge). For example, in charge operation, current is negative (flow in), and the SOC value would rise based on equation (1).
An existing method of calculating SOC is to use amp-hour integration. Due to the nature of the method, the SOC as calculated may drift from real SOC.
Background information may be found in WO06057468A1, EP1873542B1, U.S. Pat. No. 6,534,954, and US20080054850A1.